1. Field of the Invention
The invention relates generally to digital image processing and, more specifically, to a digital image processing system for enhancing the edge characteristics of printed or displayed images that are generated from low resolution image files, which include binary image data alone or a mixture of binary and gray-scale image data.
2. Description Relative to the Prior Art
Edge or line jaggedness is a common problem associated with the printing of low resolution binary image data. Efforts have been made to reduce or overcome line jaggedness using template matching methods that are applied to low resolution bitmaps as described in U.S. Pat. No. 5,005,139 or, alternatively, through the use of an outline extraction method like that described in the article entitled "Image Makers", by Henry Bortman, MacUser, November 1991, pages 98-103. Both of the above-referenced methods use a low resolution bitmap as an input file and both methods perform digital processing operations on the low resolution bitmap to produce an enhanced image output file. The enhanced output file is supplied to the printhead or a printer to produce a hard copy of the image.
There are disadvantages, however, to both of the template matching and outline extraction methods of edge enhancement discussed above. In the template matching method, the number of templates that can be employed must be limited in order to maintain acceptable processing speeds. If a pattern within the bitmap data contained in the input file does not happen to match one of the available templates, an edge resolution enhancement operation cannot be performed regardless of the quality of the image that will be produced from the input data. In the outline extraction method, variable dot sizes are used to "fill in" and match a character's outline. The fill in process, however, can be difficult when small fonts are employed. In addition, the outline extraction method tends to broaden thin graph lines that are spaced close together which results in the merger of multiple lines and a perceivable reduction in the resolution of the printed image.
Another drawback to the above-described processes is that they tend to have a negative impact on the gray scale halftone data. Thus, the processes are not particularly well suited for processing mixed image data files containing both binary image data and gray-scale halftone image data. Mixed data files, however, are becoming more and more common. Many imaging systems or multiple function devices, for example, now combine text data with gray-scale halftoned image data and binary graphic data in a single image file.
In U.S. Pat. No. 5,450,531 there is disclosed a system for performing edge enhancement of mixed low resolution image files which employs a binarization and sorting unit to sort data that is indicative of binary data from the mixed low resolution image input file. The sorted binary data is then supplied to a binary data edge enhancement processor for edge enhancement processing. The output from the binary data edge enhancement processor unit is supplied to a data merger unit along with the original image data. The data merger unit determines whether the original image data was part of a grayscale image. If the data merger unit determines that the original data is binary image data, the output from the binary edge enhancement processor unit is supplied as the output of the processing system. While the system described in this patent works well, there is a need for a system that operates with both gray level and binary printheads. There is also a need for a system that operates at higher speeds to furnish data in real time to high speed engines. There is further a need for a system that operates well with multiple function devices.
The above and other needs and objectives are accomplished in accordance with the invention described herein.